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WASHINGTON -- The Obama administration's secret legal memorandum that opened the door to the killing of Anwar al-Awlaki, the U.S.-born radical Muslim cleric hiding in Yemen, found that it would be lawful only if it were not feasible to take him alive, according to people who have read the document.

The memo, written last year, came after months of extensive interagency deliberations and offers a glimpse into the legal debate that led to one of the most significant decisions made by President Barack Obama -- to move ahead with the killing of a U.S. citizen without trial.

The secret document was narrowly drawn to the specifics of al-Awlaki's case and did not establish a broad new legal doctrine to permit the targeted killing of Americans in other circumstances. The memo provided the justification for acting despite an executive order banning assassinations, a federal law against murder, protections of the Bill of Rights and various strictures of the international laws of war, according to people familiar with the analysis.

The legal analysis relied upon several factual premises offered by intelligence agencies to government lawyers, including that al-Awlaki was playing a direct role in terrorist operations against the U.S., that he was affiliated with al-Qaida's terrorist network and that he was beyond the reach of Yemen's authorities. The memorandum, which was written more than a year before al-Awlaki was killed in a drone strike last month.

The Obama administration has refused to acknowledge or discuss its role in the strike, which technically remains a covert operation. It has also refused to release its legal reasoning. But the document that laid out its justification -- a roughly 50-page memorandum by the Justice Department's Office of Legal Counsel, completed around June 2010 -- was described on the condition of anonymity by people who have read it.

The administration did not respond to requests for comment on this article and has resisted growing calls that it provide a detailed public explanation of how it deemed the killing of a U.S. citizen to be legal. The Obama team's memorandum was principally drafted by David Barron and Martin Lederman, who were both lawyers in the Office of Legal Counsel at the time, and was signed by Barron.

The office may have given oral approval for an attack on al-Awlaki before completing its detailed memorandum. Several news reports before June 2010 quoted anonymous counterterrorism officials as saying that al-Awlaki had been placed on a kill-or-capture list around the time of the attempted bombing of a Detroit-bound airliner on Dec. 25, 2009. Al-Awlaki was accused of helping to recruit the attacker for that operation.

Al-Awlaki had also been accused of playing a role in a failed plot to bomb two cargo planes last year, part of a pattern of activities that counterterrorism officials have said showed that he had evolved from merely being a propagandist to playing an operational role in al-Qaida in the Arabian Peninsula's continuing efforts to carry out terrorist attacks.